Tire non-uniformity relates to the symmetry (or lack of symmetry) relative to the tire's axis of revolution in certain quantifiable characteristics of a tire. Conventional tire building methods unfortunately have many opportunities for producing non-uniformities in tires. During revolution of the tires, non-uniformities present in the tire structure produce periodically-varying forces at the wheel axis. Tire non-uniformities are important when these force variations are transmitted as noticeable vibrations to the vehicle and vehicle occupants. These forces are transmitted through the suspension of the vehicle and may be felt in the seats and steering wheel of the vehicle or transmitted as noise in the passenger compartment. The amount of vibration transmitted to the vehicle occupants has been categorized as the “ride comfort” or “comfort” of the tires.
Tire uniformity parameters, or attributes, are generally categorized as dimensional or geometric variations (radial run out and lateral run out), mass variance, and rolling force variations (radial force variation, lateral force variation and tangential force variation, sometimes also called longitudinal or fore and aft force variation). Uniformity measurement machines often calculate the above and other uniformity characteristics by measuring force at a number of points around a tire as the tire is rotated about its axis to generate a uniformity waveform.
Many different factors can contribute to the presence of uniformity characteristics in tires. Uniformity dispersions in tires can result from both tire harmonic effects and process harmonic effects. Tire harmonic effects or “tire harmonics” have periods of variation that coincide with the tire circumference (e.g. fit an integer number of times within the tire circumference). Tire harmonics can be attributable to tread joint width, out-of-roundness of the building drums, curing press effects, and other effects.
Process harmonic effects or “process harmonics” have periods of variation that do not coincide with the tire circumference (e.g. do not fit an integer number of times within the tire circumference). Process harmonics are generally related to process elements rather than tire circumference. Process harmonic effects can be caused, for instance, by the uniformity measurement machine itself due to non-uniformities in a road wheel used to load a tire during uniformity measurement. The manifestation of a process harmonic can change from tire to tire depending on the rate of introduction of the process harmonic relative to the tire circumference.